diff --git a/src/Composer/Downloader/ZipDownloader.php b/src/Composer/Downloader/ZipDownloader.php
index 00f7fbd1b..6217aaa53 100644
--- a/src/Composer/Downloader/ZipDownloader.php
+++ b/src/Composer/Downloader/ZipDownloader.php
@@ -56,10 +56,6 @@ class ZipDownloader extends ArchiveDownloader
             self::$hasZipArchive = class_exists('ZipArchive');
         }
 
-        if (null === self::$isWindows) {
-            self::$isWindows = Platform::isWindows();
-        }
-
         if (!self::$hasZipArchive && !self::$hasSystemUnzip) {
             // php.ini path is added to the error message to help users find the correct file
             $iniMessage = IniHelper::getMessage();
@@ -68,6 +64,15 @@ class ZipDownloader extends ArchiveDownloader
             throw new \RuntimeException($error);
         }
 
+        if (null === self::$isWindows) {
+            self::$isWindows = Platform::isWindows();
+
+            if (!self::$isWindows && !self::$hasSystemUnzip) {
+                $this->io->writeError("<warn>As there is no 'unzip' command installed zip files are being unpacked using the PHP zip extension.</warn>");
+                $this->io->writeError("<warn>This may cause invalid reports of corrupted archives. Installing 'unzip' may remediate them.</warn>");
+            }
+        }
+
         return parent::download($package, $path, $output);
     }